Radio-wave communication and navigation systems that receive signals corrupted by distributed sources of radio frequency interference (RFI) require a facility for discriminating between the desired signals and the RFI. These desired signals may be provided by a network of orbiting satellite transmitters such as the Global Positioning Satellite (GPS) system or by any terrestrial, airborne or orbiting radio frequency (RF) source(s). In particular, a fundamental liability of using the GPS system for navigational purposes is the vulnerability of GPS signals to intentional or unintentional RFI or other jamming sources. RFI that occupies frequencies outside the GPS signal band can be suppressed by filtering within the GPS receiver, whereas suppression of inband RFI requires more elaborate techniques. Accordingly, the need exists to provide an improved system for spatially discriminating against in-band RFI that emanates from directions other than the line-of-sight propagation paths from a GPS receiver to the GPS satellites, or more generally from the signal receiver to the signal source(s).
A steerable antenna array is one spatially discriminatory receiver system that is suitable for deployment in jamming environments. Ideally, the array will perform a beamforming operation that can provide gain in a desired direction (i.e., toward the GPS satellites) and attenuation of signals arriving from undesired directions (i.e., toward sources of jamming power). The beamforming operation generally involves multiplying the output from each antenna array element with a complex beam weight and then applying these weighted signals to a summing device that linearly combines the signals. The response of the beamformer is tailored to the desired reception profile by appropriately selecting the beam weights.
The most widely used RFI suppression antenna for GPS applications uses a sub-optimum mechanization in the form of a null-steering antenna array. This array is characterized by an antenna gain pattern having nulls in the direction of transmission of jammers or other sources of RFI, thus providing a form of spatial discrimination.
Despite their apparent ability to suppress RFI, null-steering antenna arrays experience a variety of operational difficulties that make them unsuited for deployment in certain signal reception environments. For example, in distributed jamming scenarios such as those in military environments, the number of individual jammer sources may well exceed the maximum number of nulls that can be formed (e.g., typically one less than the number of antenna array elements), thereby significantly reducing the ability of the receiver to suppress interference. GPS antenna arrays installed on military aircraft, for example, are typically limited by size constraints to seven antenna elements, for which one can independently suppress jamming radiation that arrives from only six directions. This limitation on the available number of antenna elements (and hence the number of nulls) is principally due to the increased degree of antenna complexity (i.e., hardware and physical installation) that accompanies each additional antenna element. A need therefore exists to develop a receiver system that is insensitive to the number and distribution of jammers.
Additionally, the deployment of null-steering configurations under certain operating conditions can cause the receiver system to experience an extended, and sometimes complete, interruption of communication services. For example, when the line-of-sight of propagation between a satellite and vehicle lies within a null profile, the null profile will not only suppress the particular RFI source(s) for which it was specifically developed, but will effectively cancel any GPS transmission from this satellite. Under these circumstances, a contingency plan may be invoked wherein the GPS receiver is forced to switch to an alternate satellite. However, this satellite switching results in a corresponding loss of service during the acquisition period for the new satellite, and is likely to cause degraded service thereafter.
In operating conditions featuring unknown or time-varying statistics for the data and/or interference signals, receiver systems have difficulty in continuously acquiring and tracking the desired signal transmissions. In order to provide a beamforming operation suitable for such conditions, null-steering arrays have been modified to incorporate an adaptive algorithm that dynamically calculates the beamforming weights so that the beamformer response converges to a statistically optimum nulling solution.
In this adaptive array configuration, the outputs of auxiliary antenna elements are weighted and combined with the output of a primary antenna element to minimize the total received signal power through the appropriate selection of the beamforming weights. The underlying premise of this adaptive scheme is that the desired signal (e.g., GPS information) is weak with respect to interference; otherwise, the jammer would be ineffective. Therefore, by minimizing input power, the signal level of the interference sources is minimized. This minimization has the effect of forming "nulls" in the antenna gain pattern direction to the strongest interference source(s). The combining (i.e., beamforming) weights are continuously adjusted to account for relative motion of the vehicle with respect to the signal sources, such that the nulls remain aligned with the desired interference sources.
However, as a result of such minimization, the desired signal may be cancelled or significantly attenuated. This limitation appears in any type of GPS power minimizer and hence makes the adaptive array an inadequate device for achieving precise data acquisition.
Another feature of this adaptive approach is that multiple iterations of signal sampling and beam formation are typically required in order to converge upon the optimum null-steering weights. Although non-iterative techniques have been used to implement the statistically optimum beamformer, iterative solutions have received the most attention since a non-iterative approach relies upon making measurements of incoming signals that may not sufficiently converge within the time period (e.g., tens of milliseconds) typically necessary to adequately track the signal.
The iterative adaptive algorithm has been shown to be successful in achieving signal lock during stationary operation; however, its effectiveness is reduced by motion of the vehicle platform (especially rotational) that houses the receiver system. For example, with approximately 10 ms being necessary to converge to the desired pattern (a property that appears in virtually all existing antenna implementations), there is insufficient time available for the adaptive algorithm to properly form nulls when the vehicle platform is continuously turning. In particular, the adaptive algorithm cannot adjust the beamforming weights fast enough to accommodate rapid changes in the attitude of the host vehicle.
Furthermore, with intelligent jamming such as blinking jammers, the algorithm must develop nulling profiles for a jammer network whose spatial distribution is constantly changing. Accordingly, since an intelligent adversary will cause the distribution of blinking jammers to change at a rate faster than the convergence time of the null-forming algorithm, the dynamic nature of the jammer network prevents the algorithm from converging upon optimum beamforming weights and thereby renders the algorithm virtually ineffective.